High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly (ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
It has been previously disclosed in co-pending application Ser. No. 662,910 filed Mar. 1, 1976, assigned to the same assignee as herein, that glass reinforced thermoplastic compositions of a polycarbonate resin and poly (1,4-butylene terephthalate) can be molded to articles having greater resistance to warpage or heat distortion, in comparison with glass fiber reinforced poly(1,4-butylene terephthalate) resins.
It has now been discovered that blends of a poly (1,4-butylene terephthalate) resin, a poly(ethylene terephthalate) resin and a polycarbonate resin, reinforced with fibrous glass, possess less inherent warpage in the molded article and good moldability when compared with compositions of glass fiber reinforced poly(1,4-butylene terephthalate). The improved resistance to heat distortion is achieved without any appreciable decrease in other mechanical properties, such as heat resistance (as measured by the heat deflection temperature under load), notched Izod impact strength, tensile strength, modulus and flexural strength. In addition, the present compositions are less susceptible to increases in melt viscosity than the above-mentioned glass reinforced blends of poly(1,4-butylene terephthalate) and polycarbonate resins.